1. Field of the Invention
The present invention relates to a vehicle wet-type multi-plate brake device which is applicable to various vehicle brake devices including industrial vehicle brake devices and in which a mating plate attached to a frame support connected to an axle housing is configured to engage with or disengage from a friction plate attached to a brake hub connected to an axle shaft, and more particularly to, a vehicle wet-type multi-plate brake device provided with an automatic control mechanism for constantly maintaining a piston stroke of a hydraulic piston upon operating a brake.
2. Description of the Related Art
Among brake devices used for wheeled industrial vehicles such as a forklift and a vehicle for a specialized operation, a wet-type multi-plate brake is widely used in which a mating plate is configured to engage with or disengage from a friction plate by the use of a hydraulic piston.
In such a wet-type multi-plate brake, when the mating plate or the friction plate is abraded, a piston stroke of the hydraulic piston is changed, thereby causing a problem in that a braking time or a non-braking time after a brake operation is changed, a driving feeling is deteriorated, and then a necessary oil quantity is increased.
For this reason, as the conventional solution methods, there are disclosed a method of constantly maintaining a piston stroke by controlling a quantity of operation oil itself supplied to a hydraulic piston, a method of constantly maintaining a piston stroke by the use of a returning force of a sliding resistance produced in a piston seal of a hydraulic piston, and the like.
However, in the method of controlling the supplied oil quantity, it is disadvantageous in that an attachment space and a cost increase largely due to an increase in size of a device. Additionally, since there is a limitation in regulation amount in a stroke regulation using the piston seal, when the number of brake plates increases, a problem arises in that drag or the like occurs.
Additionally, a multi-plate brake device mounted with an automatic control mechanism for constantly maintaining a piston stroke of a hydraulic piston upon operating a brake is disclosed in, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2000-304078).
As shown in FIG. 4, the technique disclosed in Patent Document 1 has a configuration in which an annular piston 04 is provided in order to operate a brake in such a manner that a disk-shape inner disk 02 and an annular outer disk 03 constituting a disk brake 01 engage with each other in a friction state, and a hydraulic pressure is supplied to an oil chamber 05 provided in rear of the piston 04 upon operating the brake.
Further, a hole 07 is provided so as to be opened toward an extending portion 06 of the piston 04, and a sleeve 08 is fitted to the hole 07 in a friction-engage state. Then, a bar 09 is inserted through a center portion of the sleeve 08 with a gap therebetween in a circumferential direction. One end of the bar 09 is screw-connected to the extending portion 06 of the piston 04 by a screw-connecting portion 010. Additionally, the other end of the bar 09 is provided with a head portion 011 opposed to the inner end surface of the sleeve 08 with a predetermined gap A therebetween.
Then, a sliding resistance produced by the friction-engagement between the hole 07 and the sleeve 08 is set to allow the sleeve 08 to advance when the piston 04 advances again in a state where the piston 04 advances by the predetermined gap A in terms of a hydraulic action and the head portion 011 engages with the end surface of the sleeve 08, but is set not to allow the sleeve 08 to recede when the piston 04 is receded by a return spring 012 upon removing the hydraulic pressure.
Accordingly, the sleeve 08 displaces in an advancing direction by the friction amount produced by the disks 02 and 03, and the gap A is constantly maintained all the time. For this reason, even when the disk 02 and 03 are abraded, the stroke of the piston 04 is constantly maintained all the time.
However, the automatic control mechanism for controlling the piston stroke of the hydraulic piston disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2000-304078) has the following problems to be solved.
Since the piston 04 is configured as one annular member, the contact surface of the piston 04 against the disks 02 and 03 is easily distorted. Accordingly, the motion of the piston 04 using the hydraulic pressure supplied to the oil chamber 05 cannot act uniformly in an axial direction. Also, the motion of the piston 04 cannot be high-precisely carried out in consideration of the abrasion states of the disks 02 and 03. For this reason, it is difficult to obtain the automatic control mechanism in which the movement stroke of the piston 04 is high-precisely set in consideration of the abrasion states of the disks 02 and 03.
That is, in order to maintain the predetermined gap A shown in Patent Document 1, the initial position of the sleeve 08 is set by the returning force of the return spring 012. However, since the return spring 012 urges the outer disk 03, urges the piston 04, and then urges the sleeve 08 via the piston 04, the initial position of the sleeve 08 is unstably determined upon returning the return spring 012, thereby causing a problem in that the stroke cannot be controlled with high precision.
Additionally, since the return spring 012 acts on the outer disk 03, when a return spring having a large spring constant is set in order to reliably return the piston 04, a force of the return spring increases with the abrasion states of the brake disks 02 and 03, thereby causing a problem in that a driving feeling is deteriorated.
Then, since an axial force of the sleeve 08 is controlled by the fitting state, it is difficult to carry out processing and assembling operations. Also, when the abrasion occurs in age, a function is damaged.